The invention relates to a process for preparing N-acylamino acid esters and N-acylamino acetals.
A large number of different methods for synthesizing amino acids and their esters are known. A review is given, inter alia, in Ullmanns Encyclopedia of Industrial Chemistry, Vol. A2, 57-97, VCH Weinheim 1985.
Industrial syntheses of D,L-xcex1-amino acids, for example the Strecker synthesis, use aldehydes as starting materials, which are reacted with NH3 and HCN to give aminonitriles. The nitrile group can subsequently be reacted with alcohols or water to give the corresponding esters and amino acids, respectively.
DE-A-3145736 describes a process for preparing N-formyl-xcex1-amino acid esters by reacting aminonitrilesxe2x80x94for example from the Strecker synthesisxe2x80x94with an appropriate alcohol and formamide in the presence of hydrogen chloride.
Also known is the preparation of N-formyl-D,L-alanine from pyruvic acid by boiling with ammonium formate in formic acid [F. Yoneda and K. Kuroda, J. Chem. Soc. Chem. Commun., 1982, 927-929].
N-Formylalanine esters are used, inter alia, for preparing vitamin B6 (Pyridoxine) [Review by Kxc3x6nig and Bxc3x6ll, Chem. Ztg. 100 (1976), 107/8] and isocyanic acid, for example according to Ugi, Angew. Chem. 77 (1965), 492.
The processes described have the disadvantage that the starting materials used are finished amino acids or precursors thereofxe2x80x94for example cyanohydrins or aminonitriles from the Strecker synthesisxe2x80x94which have to be prepared beforehand in a separate process.
It is an object of the present invention to provide a process for preparing N-acylamino acid esters and N-acylamino acetals which can easily be carried out on an industrial scale, using readily-obtainable starting materials.
We have found that this object is achieved by a process for preparing N-acyl derivatives of the formula I 
in which the substituents independently of one another have the following meanings:
X is CH(OR3)2, COOR3;
R1 is hydrogen, C1-C12-alkyl, aryl, unsubstituted or substituted;
R2 is hydrogen, C1-C12-alkyl, aryl, unsubstituted or substituted;
R3 is C1-C12-alkyl,
which comprises reacting a carboxamide R1xe2x80x94CONH2 of the formula II with a glyoxal monoacetal derivative of the formula III, 
xe2x80x83in the presence of a carboxylic acid R4xe2x80x94COOH of the formula IV where R4=C1-C12-alkyl, where the substituents R1 to R3 are as defined above.
Alkyl radicals for R1 to R4 which may be mentioned are branched or straight-chain C1-C12-alkyl chains, for example methyl, ethyl, n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2 methylpropyl, 1,1-dimethylethyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl, 1-ethyl-2-methylpropyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, n-undecyl and n-dodecyl.
The alkyl chains mentioned above can be unsubstituted, hydroxylated or substituted by mercapto groups. Preferred examples which may be mentioned are hydroxymethyl, hydroxyethyl, such as [CH3xe2x80x94CH(OH)xe2x80x94 or CH2(OH)xe2x80x94CH2] or mercaptomethyl radicals.
If the radical X in the formula I is CH(OR3)2, the substituents R3 together with the oxygen atoms to which they are attached may also form a 5- or 6-membered ring. Starting materials used in this case are, for example, cyclic glyoxal monoacetals of the general formulae IIIa to IIIc. 
Aryl for R1 and R2 is to be understood as an aromatic ring or ring system having 6 to 18 carbon atoms in the ring system, for example phenyl or naphthyl, which may be unsubstituted or substituted by one or more radicals, such as halogen, for example fluorine, chlorine or bromine, cyano, nitro, amino, C1-C4-alkylamino, C1-C4-dialkylamino, hydroxyl, C1-C4-alkyl, C1-C4-alkoxy or other radicals.
Preferred radicals for R1 are hydrogen and the branched or straight-chain C1-C8-alkyl chains mentioned in the list above, particularly preferably C1-C3-alkyl chains. Very particularly preferred radicals for R1 are hydrogen, methyl and ethyl.
Preferred radicals for R2 are phenyl and the branched or straight-chain C1-C8-alkyl chains from the list mentioned above, particularly preferably C1-C3-alkyl chains. A very particularly preferred radical for R2 is methyl.
Preferred alkyl radicals for R3 are the branched or straight-chain C1-C8-alkyl chains from the list mentioned above, particularly preferably C3-C8-alkyl chains, such as, for example, n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2-methylpropyl, 1,1-dimethylethyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, n-heptyl, n-octyl or 2-ethylhexyl.
Preferred radicals for R4 are the branched or straigh-chain C1-C8-alkyl chains from the list mentioned above, particularly preferably C1-C3-alkyl chains. Very particularly preferred radicals for R1 are methyl, ethyl, n-propyl and isopropyl.
Depending on the amount of carboxamide R1xe2x80x94CONH2 and carboxylic acid R4xe2x80x94COOH employed, the formation of the different N-acyl derivatives of the formula I can be controlled in a targeted manner.
Thus, contrary to expectation, it has been found that reaction of an amount of carboxamide R1xe2x80x94CONH2 and carboxylic acid R4xe2x80x94COOH employed of in each case from 250 to 800 mol %, preferably from 400 to 600 mol %, based on the acetal of the formula II employed, gives N-acylamino acid esters of the formula I where X=COOR3.
A particularly advantageous embodiment of the process was found to be the use of the carboxamide R1xe2x80x94CONH2 and the carboxylic acid R4xe2x80x94COOH in identical molar proportions.
The process according to the invention is particularly suitable for preparing N-formyl-xcex1-aminopropionic acid esters of the formula Ia 
in which the substituent R3 is C1-C8-alkyl, preferably C3-C8-alkyl.
Formation of the N-acylamino acetals of the formula I where X=CH(OR3)2 is preferred when the amount of carboxamide R1xe2x80x94CONH2 and carboxylic acid R4xe2x80x94COOH employed is in each case from 50 to 250 mol %, preferably from 100 to 200 mol %, based on the acetal of the formula II employed. In this case, too, it is particularly advantageous to employ carboxamide R1xe2x80x94CONH2 and carboxylic acid R4xe2x80x94COOH in the reaction in a molar ratio of 1:1.
In the case of the N-acylamino acetals of the formula I, the process according to the invention is advantageously suitable for preparing N-formyl-2-aminopropionaldehyde acetals of the formula Ib 
in which the substituent R3 is C1-C8-alkyl, preferably C3-C8-alkyl.
The conversion both into the N-acylamino acid esters and into the N-acylamino acetals is carried out at from 40 to 200xc2x0 C., preferably from 60 to 150xc2x0 C.
According to the invention, the reaction is carried out in a pressure range of from 200 to 1000 mbar, preferably between 500 and 1000 mbar, particularly preferably at atmospheric pressure.
The reaction can be carried out with or without additional solvent. The reaction is preferably carried out without adding a solvent.
Moreover, the process according to the invention can be carried out advantageously as a xe2x80x9cone-pot processxe2x80x9d, giving both N-acylamino acid esters and the novel N-acylamino acetals in excellent yields.
The isolation of the desired end product is carried out in a manner known per se. In the case of liquid reaction products, the esters or acetals formed are usually purified by distillation.
The invention also provides N-acyl derivatives of the formula Ic, 
in which the substituents independently of one another have the following meanings:
R1 is hydrogen, C1-C12-alkyl, aryl, unsubstituted or substituted;
R2 is hydrogen, C1-C12-alkyl, aryl, unsubstituted or substituted;
R3 is C1-C12-alkyl.
Preference is given to N-acyl derivatives of the formula Ic, in which the substituents independently of one another have the following meanings:
R1 is hydrogen, C1-C8-alkyl;
R2 and R3 are C1-C8-alkyl.
With respect to the exact definition of the substituents R1 to R3, both in the general and the preferred embodiments, the definitions given at the outset for the compound I should be referred to.
The N-acylamino acetals of the formula Ic are suitable for use as intermediates for preparing oxazoles.